This invention relates generally to filled polymeric composites. More particularly, this invention relates to a polymeric composite for use as an electrical substrate material which exhibits both a high dielectric constant (K') and a low thermal coefficient of dielectric constant (TCK'). This electrical substrate material is particularly useful as a laminate for manufacturing microwave circuits.
The electrical performance of electrical circuits and devices is highly dependant on the dielectric constant, K', of the dielectric medium. Thus, when the dielectric constant of a material changes with temperature, the electrical performance of the device will change as well.
A basic treatment of the factors affecting the temperature coefficient of dielectric constant (TCK') of homogeneous compounds is discussed in detail in U.S. patent application Ser. No. 08/099,245 now U.S. Pat. No. 5,358,775, which is assigned to the assignee hereof, all of the contents of which are incorporated herein by reference.
Many polymeric composite materials are presently available for use as a laminate for microwave frequency electronic applications. Prevalent amongst these materials are composite systems based on PTFE (poly(tetrafluoroethylene)) and other fluoropolymers (such as FEP (poly(tetrafluoroethylene-co-hexafluoropropylene) and DuPont's PFA). Fluoropolymeric composites are desirable due to their excellent high-frequency electrical properties and excellent high temperature and solvent resistance. When referring to organic polymer based composite circuit substrate, low K' generally denotes a K' value of less than 3.0, while high K' implies a value of greater than about 4.0.
A common class of fluoropolymeric composite microwave laminates are those that are reinforced in the XY plane with either woven glass cloth or random glass microfiber. Examples of such materials are Rogers Corporation's RT/duroid.RTM. 5880 (e.g., random glass), and Ultralam.RTM. 2000 (woven glass) and the material described in U.S. Pat. No. 4,886,699, assigned to the assignee hereof and incorporated herein by reference. The dielectric constant values of these types of materials commonly ranges from 2.17 to about 2.65. This class of materials exhibits comparatively high Z-axis coefficients of thermal expansion (CTE), ranging from +125 to +250 ppm/.degree.C. In spite of the high Z-axis CTE of these materials, the thermal coefficient of K' is relatively low. The TCK' of RT/duroid 5880 has been measured to be approximately -75 ppm/.degree.C. at a frequency of 10 GHz over the temperature range of 20.degree. C. to 250.degree. C. This comparatively good TCK' is due in part to the relatively low K.degree. of this class of materials.
Another type of fluoropolymer composite useful in microwave laminates is described in U.S. Pat. No. 4,849,284, assigned to the assignee hereof and incorporated herein by reference. A preferred embodiment of this invention is sold by Rogers Corporation to the microwave circuit board industry under the trademark RT/duroid.RTM. 6002. This composite material consists of fused amorphous silica, PTFE and E-glass microfibers. It exhibits a K' of 2.94, a Z-axis CTE of about 24 ppm/.degree.C. and a TCK' of about +20 ppm/.degree.C. The small value for the TCK' allows for stable electrical performance of circuits made on RT/duroid 6002 over a wide range of temperature. This feature is highly valued by designers of microwave circuits.
This and other materials made with a similar formulation, however, are not available with a K' of greater than about 3.0, due to the comparatively low K' (3.78) of the silica filler.
It is known that there is also a need for comparatively high K' (K'.gtoreq.5) fluoropolymeric composite materials such as those described in U.S. Pat. No. 4,335,180, (which is assigned to the assignee hereof and incorporated herein by reference). Such high K' materials are sold by Rogers Corporation under the trademark RT/duroid 6006 (K'=6.15) and RT/duroid 6010 (K' of 10.2 to 10.8). This class of materials is generally made by adding titania ceramic filler to increase the K' of the material. These high K' materials exhibit z-axis CTE's of about +45 ppm/.degree.C. to +80 ppm/.degree.C. and TCK' values of about -500 ppm/.degree.C. to -600 ppm/.degree.C. The major factor in causing the high TCK' of this class of materials is the high TCK' of the ceramic filler that is added to modify the K' (although as is demonstrated in the examples below, it has been discovered by the inventor herein that the CTE also plays a major role). Titania (TiO.sub.2) itself exhibits a TCK' of approximately -750 ppm/.degree.C.
It is clear from the discussion above that the prior art high-K' composite laminate materials do not exhibit a low TCK'. Some recently developed materials, however, simultaneously achieve these two properties.
One such material is TMM.RTM. temperature stable microwave laminate sold by Rogers Corporation and described in detail in U.S. Pat. No. 5,223,568, which is assigned to the assignee hereof and incorporated herein by reference. TMM is a ceramic powder filler composite made with a thermoset (e.g., polybutadiene) matrix. TMM exhibits a low TCK' and dielectric constants of 3 to 13 depending on the type of ceramic filler used. The high-K' formulations for the TMM material requires the use of fillers that exhibit both a high K' (greater than about 30) and a low TCK'.
Ceramic materials that exhibit high-K' and low TCK' (known as NPO ceramics) are generally proprietary formulations that are made for the capacitor industry. As such, these materials are usually comparatively expensive, with prices of greater than $35/kg. Since these materials are also usually very dense (Sp.G. &gt;5.0), the relative price on a volumetric basis can be even higher when compared to other ceramic fillers. A further drawback of some high-K'/low-TCK' ceramic fillers is that they contain the element lead. Though the lead is expected to be in an insoluble and harmless form, there are uncertainties and legal concerns associated with the lead content. Some of these NPO ceramic materials also exhibit unacceptably high dielectric loss when tested at frequencies of greater than 500 MHz.
The ceramic filled PTFE composite microwave circuit substrates described in aforementioned U.S. Pat. No. 5,358,775 are also formulated to exhibit both a high-K' and comparatively low TCK'. The formulations require a low TCK' (absolute value of less than 150 ppm/.degree.C.) ceramic filler of the type described above (e.g., NPO ceramic) that exhibits a K' of greater than about 30 and a second filler that exhibits a low coefficient of thermal expansion. The drawbacks inherent in using high-K'/low-TCK' ceramic materials of this type, particularly the associated high cost, are discussed above.